Design: Making Gambrels Work

When using modern methods and materials, this traditional coastal vernacular requires a little structural translation

The gambrel roof style has a long history of enduring the elements.
Some of the longest-standing wood-frame houses in America have
gambrel roofs that date from the 1600s (see the photo below for the
consummate example). I see a lot of gambrel roofs on Victorian-era
houses in towns along the eastern seaboard (although the slightly
different "mansard" variation is a more typical coastal style), and
I also find gambrels on many shingle-style seaside summer homes
built in the early part of this century.

Considered the oldest wood-framed house in North America, the
John Fairbanks house in Dedham, Mass., dates to 1636. The two
gambrel additions were completed later, probably around
1668.

In new construction, the style lives on into modern times, adapted
slightly for platform framing methods. Even gambrel roof trusses,
including space trusses that enclose usable attic space, are
commonly available today, although when I use the gambrel form I
prefer to stick-frame it.

Practical Advantages

What accounts for the gambrel's enduring popularity? For me, the
gambrel is a practical and attractive way to create usable space
under a roof.

I've always felt that houses look better when the roofline springs
from the first-story wall plate, rather than from the second story.
The cape style, of course, is the classic example. But a cape roof
can rob usable space from the upstairs floor plan. A gambrel gives
you back that room, without having to form huge dormers.

The floor plan isn't the only issue, however. A cape roof with a
full shed dormer provides little or no space for ductwork. (Capes,
in my experience, are a nightmare to air condition.) A gambrel
roof, on the other hand, not only gives you very nearly a full
second story, but it also creates a small attic in the upper,
triangular portion of the roof, where you can run wires and
ductwork. If a gambrel fits in well with the historical character
of the area in which you're building, then it's a win all
around.

Style Meets Structure

A roof, however, is never all about aesthetics or just about
enclosing space. There are significant structural issues, too, and
while old timber gambrels have survived, the form doesn't translate
perfectly to dimensional lumber. One gambrel rendition, for
example, that dates from the late 1800s and early 1900s, relies on
a complex assembly with a lower, steep-pitch set of rafters
springing from the wall plate up to a 4x6 horizontal nailer at
midroof (Figure 1). A knee wall supports the lower portion of these
rafters but places a sizable point load on the floor joists.

Figure 1. Late 19th-century builder guides
show this gambrel framing concept, which the author has emulated
using modern materials. Taken together, the structural impacts
— the load of the upper roof, the upward reaction of the knee
walls, and the restraint at the main wall plates — place the
lower rafters in bending. The knee walls transmit part of the
roof's weight onto the floor joists, which must be designed to
handle that point loading.

I have built gambrels using a similar design, but I rely on
upper-story web floor trusses to carry the point loads. Yet even
with trusses for joists, the result is still not entirely
satisfactory — there is enough minor movement in the floor
system to create a small amount of finish cracking in the rooms
below.

Another example found in one old house near the shore that I
remodeled shows how the gambrel was understood in the days of
balloon framing. In this case the upper portion of the main house
walls served as the knee wall for the space upstairs (Figure 2).
Orig-inally built as a summer cottage in the late 1800s, this house
was a little underframed even for that era, and the movement of the
roof betrayed some of the flaws in the concept.

Figure 2. In a late-1800s gambrel the author
remodeled, floor joists were attached to the side of the main wall
studs, resting on a 1x6 ledger board let into the studs. The upper
portions of the stud walls served as knee walls in the upstairs
rooms. The outward thrust of the lower gambrel rafters caused the
wall plates to sag when the floor-to-wall connections slipped. And
the nailer where the upper and lower rafters meet, which was too
small to serve as a beam, had also sagged.

For the roof to work, the upper part of the balloon-framed walls
would have had to act as a cantilever, resisting the thrust of the
lower rafters. But the main floor joists weren't well attached to
the wall — they were nailed only to the 1x6 let-in ledger
board rather than securely attached to the wall studs. The wall
plate had bowed horizontally, and as the studs pushed outward, some
of the joists had slipped out of position and dropped. The 4x6
nailer at the midroof joint had also sagged (although it had some
end support, it was too small to function as a beam).

I managed to preserve most of that structure by reinforcing
connections and by shoring up the roof rafters with new partitions,
closet framing, and posts here and there. That roof had been there
a hundred years, and it will probably go another hundred now. But I
also saw that if I were to frame a new gambrel, I wouldn't want to
emulate that example.

A Rigid Frame

When I built my own house, I designed the gambrel roof as a
full-span rigid frame. Instead of a horizontal nailer where the
sawn-lumber rafters met, I connected them with 3/4-inch plywood
gussets (Figure 3). Each midspan rafter knee is a rigid connection.
Because the joints are rigid, the loads on the roof induce a lot of
bending force in the rafters; therefore, I used 2x12s that could
fully resist the bending. (The 2x12s gave me lots of room for
insulation, too.)

Figure 3. Using a rigid gambrel frame made
with 2x12 rafters and glued and nailed plywood gussets, the author
was able to get a 28-foot clear span. For gravity loads, the system
is analyzed as a triangular "three-hinged arch" — the ridge
connection and the two rafter-to-plate connections are thought of
as hinge connections, while the midspan gussets are treated as
rigid joints. The birdsmouth at the floor plate helps transfer a
moderate horizontal thrust to the floor diaphragm, while the
gussets joining the rafters at the roof peak help stabilize the
structure under unbalanced wind-load conditions.

This is an unusual solution, and the loads were tricky to analyze,
but there's a considerable factor of safety in the oversized sawn
lumber and in the heavily nailed gussets. The roof is performing
fine in service, and I get all the benefits of the gambrel —
plenty of usable space with the aesthetic appeal of a roof that
springs from the first floor.

Architect and builder Andrew DiGiammo owns and
operates a custom design/build firm based in Assonet,
Mass.